We consider a multi-class G/G/1 queue with a finite shared buffer. There is task admission and server scheduling control which aims to minimize the cost which consists of holding and rejection components. We construct a policy that is asymptotically optimal in the heavy traffic limit. The policy stems from solution to Harrison-Taksar (HT) free boundary problem and is expressed by a single free boundary point. We show that the HT problem solution translated into the queuelength processes follows a specific {\it triangular} form. This form implies the queuelength control policy which is different from the known $cμ$ priority rule and has a novel structure. We exemplify that the probabilistic methods we exploit can be successfully applied to solving scheduling and admission problems in cloud computing.